This invention relates generally to printed circuit boards, and more particularly, but not by way of limitation, to dissipation of heat in printed circuit boards.
Integrated circuit chips (ICs) for controlling electronic devices are currently cut from silicon wafers and packaged so that they can be electrically attached to circuitry of a printed circuit board (PCB). The top surface of the PCB typically includes electrical contacts to which terminals of the IC package may be connected.
PCBs are typically made of layers of an insulating substrate material interleaved with signal layers, which include traces connecting PCB-mounted components to other components. The use of multiple signal layers increases the ability to route these traces along the PCB, because they are able to pass under or over components and other traces of the PCB, rather than around them. Ground and power planes are typically interspersed between these signal layers. ICs and other components coupled to the PCB may tap their ground and power sources from these ground and power planes.
In multi-layer PCBs, it is known to route electrical signals from layer to layer by through-holes, or vias. Vias are holes which extend through the PCB layers and are typically internally lined with conductive material to electrically connect traces or mounting contacts to another circuit board layer. Openings for these signal vias are usually formed by mechanically punching any one or many PCB layers prior to PCB lamination.
As technology has moved forward, ICs have been designed to carry out more functions of greater complexity. As a result, the number of electrical contact points for power supply and input-output signals to and from ICs continues to increase. IC packages that can handle an increased number of electrical contact points are therefore required. Ball grid array (BGA) chip packaging, which utilizes solder balls on its mounting surface instead of pins for mounting to PCB contacts, are especially useful because they allow for more contacts per unit package area.
However, faster, more complex Ics rely on BGAs to dissipate more power than. their technological predecessors. If a sufficient amount of heat generated by an IC is not removed, its performance may be degraded, and the chip may even be destroyed as a result. Therefore, it is necessary to design heat removal systems into computer systems that use high-speed ICs.
Many effective methods have traditionally been used for removing this heat. For example, fans can be provided for generating airflow. However, fans require additional costs as well as additional space no longer available in electronic devices which are designed to be ever smaller. As another example, heat sink layers can be added to the IC package substrate, the PCB, or both. However, these additional layers also give rise to additional costs associated with both materials and manufacture.
One inexpensive way in which heat has been removed from ICs has been to conduct it into a preexisting heat sink layer of the PCB. A ground plane, for example, is made of a highly conductive material such as copper, extends throughout the PCB and may therefore be effective in dissipating heat. Because the PCB already includes a ground plane, this arrangement does not require the costs associated with providing additional heat sink layers. Heat can be conducted into the ground plane from a BGA package through ground vias in the center of the BGA substrate directly below the IC. These ground vias are connected to solder ball terminals on the lower surface of the BGA package for connection to contacts on the upper surface on the PCB. Corresponding ground vias are also provided in the PCB so as to connect the contacts to the ground plane. Because these ground vias are designed to conduct heat, they are also known as xe2x80x9cthermalxe2x80x9d vias. These thermal vias have been effective in removing heat from ICs to a certain extent.
However, as ICs have continued to increase in complexity, thereby requiring even more power for operation, it has been found that mere provision of thermal vias is no longer sufficient for adequate heat removal. This is in part because the preexisting heat sink layers of the PCB are perforated by holes which decrease their effectiveness in dissipating heat. These holes are necessary in order to allow signal vias to extend through the heat sink layers while remaining insulated from them. In the case of a BGA package, the holes associated with signal vias surround the area of the heat sink layer connected to the thermal vias, thereby preventing effective transfer of heat from the area beneath the IC. What is needed is a way to increase transfer of heat away from the IC without incurring the additional costs associated with traditional heat-removal methods.
The present invention is an improved PCB layout for increasing the ability of the PCB to transfer heat away from a component mounted thereon. Signal vias in the PCB are positioned so as to define pathways to a surrounding heat sink layer of a PCB. This allows heat to be more effectively conducted away from thermal vias to the heat sink layer. Additional features and benefits will become apparent upon a review of the attached figures and the accompanying description.